Stokes IQUV magnetic Doppler imaging of Ap stars - I. ESPaDOnS and NARVAL observations

In this paper we describe and evaluate new spectral line polarization observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete or partial Stokes IQUV sequences, corresponding to 297 individual polarized spectra, have been obtained for seven bright Ap stars using the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) and NARVAL high-resolution spectropolarimeters. The targets span a range of masses from approximately 1.8 to 3.4 M circle dot, a range of rotation periods from 2.56 to 6.80 ?d and a range of maximum longitudinal magnetic field strengths from 0.3 to over 4 kG. For three of the seven stars, we have obtained dense phase coverage sampling the entire rotational cycle. These data sets are suitable for immediate magnetic and chemical abundance surface mapping using magnetic Doppler imaging. For the remaining four stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km?s-1 pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50-100 sigma level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. In addition to providing the foundation for the next generation of surface maps of Ap stars, this study establishes the performance and stability of the ESPaDOnS and NARVAL high-resolution spectropolarimeters during the period 2006-2010.